Silver-containing thermographic and photothermographic imaging materials (that is, thermally developable imaging materials) that are imaged and/or developed using heat and without liquid processing have been known in the art for many years.
Silver-containing thermographic imaging materials are non-photo-sensitive materials that are used in a recording process wherein images are generated by the use of thermal energy. These materials generally comprise a support having disposed thereon (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing composition (usually including a developer) for the reducible silver ions, and (c) a suitable hydrophilic or hydrophobic binder.
In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. At elevated temperatures, the silver of the silver carboxylate is reduced by a reducing agent for silver ion such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechols, pyrogallol, ascorbic acid, and ascorbic acid derivatives, whereby an image of elemental silver is formed. Some thermographic constructions are imaged by contacting them with the thermal head of a thermographic recording apparatus such as a thermal printer or thermal facsimile. In such constructions, an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized. The resulting thermographic construction is then heated to an elevated temperature, typically in the range of from about 60 to about 225° C., resulting in the formation of an image.
Silver-containing photothermographic imaging materials (that is, photosensitive thermally developable imaging materials) that are imaged with actinic radiation and then developed using heat and without liquid processing have been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation (for example, X-radiation, or ultraviolet, visible, or infrared radiation) and developed by the use of thermal energy. These materials, also known as “dry silver” materials, generally comprise a support having coated thereon: (a) a photocatalyst (that is, a photosensitive compound such as silver halide) that upon such exposure provides a latent image in exposed grains that are capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a relatively or completely non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a hydrophilic or hydrophobic binder. The latent image is then developed by application of thermal energy.
In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced.
In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a “developer,” may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for photothermographic materials. At elevated temperatures, the reducible silver ions are reduced by the reducing agent. This reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the photothermographic imaging layer(s).
Problem to be Solved
It is frequently desirable to be able to provide an indication whether a product has been exposed to an undesirable time-temperature history that results in substantial degradation of the product or to correct the time-temperature history during processing or use of the product. It may also be desirable to provide an indication whether a product has been exposed to an undesirable temperature, no matter what the length of exposure that results in substantial degradation of the product or complete product failure.
The rate of degradation at a given temperature is typically product dependent, that is, some types of products show a greater increase in the rate of change for a given temperature increase relative to other products. Moreover, certain types of products experience a greater rate of degradation after being exposed to, for no matter how briefly, an elevated temperature. Accordingly, it would be desirable to be able to have a visual indication of both cumulative thermal exposure and a peak thermal event exposure in which the rate of providing the visual indication of cumulative thermal exposure and thermal event exposure can be approximately matched to the rate of change, such as degradation, of the specific product to be monitored.
Thermally developable materials including photothermographic and thermographic materials are commercially available from several manufacturers and are usually packaged in sheet form with multiple sheets (perhaps 50 or more) in a light-tight plastic wrap or metal or plastic container or cartridge that is usually in a light-tight cardboard box. The sheets are then fed into an imaging and/or processing apparatus from the package or container as needed.
Packaged thermally developable materials are stored, transported, or used under a variety of environment conditions and thus they are exposed to various humidity and temperature conditions. Despite the fact that the individual sheets are usually in the light-tight packaging wrap exposure to environmental conditions such as heat over time, may adversely impact the performance of the materials and degrade their usefulness or shelf life.
It would be desirable to be able to provide an indication whether a particular thermally developable material has been exposed to an undesirable time-temperature history that results in substantial degradation of the product or to provide a time-temperature history that may be required during processing or use of the product. It may also be desirable to determine whether a thermally developable material has been exposed to a single temperature event.
A number of systems have been described for providing indicators useful in detecting whether a product has been exposed either to specific time-temperature combinations or simply to a particular temperature. For example, U.S. Pat. No. 3,999,946 (Patel et al.) describes the use of compositions containing at least two conjugated acetylene groups that exhibit sequences of irreversible color changes at combinations of times and temperatures.
Indicators for temperature-time exposure of various packaged photographic imaging products have been developed for several years as described in U.S. Pat. No. 6,113,857 (Patel et al.) and WO 96-28714 (Arens et al.). Indicators for providing information about the cumulative impact of thermal exposure also have been developed for canisters of wet-processed color photographic film as described in U.S. Pat. No. 5,940,637 (Manico et al.), U.S. Pat. No. 6,043,021 (Manico et al.), U.S. Pat. No. 6,103,351 (Ram et al.), and U.S. Pat. No. 6,113,857 (Manico et al.).
Because of the unique nature of the imaging chemistry and/or thermal developing conditions in thermally developable materials compared to conventional wet-processed photographic films, there is a need to provide a means for detecting the temperature-time history and single temperature events for packaged sheets of thermally developable materials.